30 CHEMICAL ENGINEERING
WS(Q' - Q) -
Where, Qf = Heat content of superheated steam at temperature £'i, B.t.u. per
Q — Heat content of saturated steam, B.t.u. per pound, and tm - (T'« + T'i -t' - 0/2.
In ordinary practice, for setting-type superheaters, k = 2 to 4 : for independently fired, k = 8 to 12. If set as prescribed, (7"2 -f- T'i) will be not far from twice (tr -f 0-The quantity Q' — Q is approximately equal to 0.5 (tr — t). The ratio of Wo to Ws is fixed as in economizers.
Condensers. — From a thermal standpoint, condensers may be classified as mixing (horizontal independent jet, injector or barometric) and non-mixing (surface) types. The equations above given for feed- water heaters will apply: as an error on the safe side, x may be taken as 1.0. The value of t\ is determined by the available supply of cooling water: in this latitude under summer conditions, it is rarely below 70°. That of t0 will be in surface condensers 5 to 10° below ty and that of ti the same amount below fo. The value of £0 determines the best possible vacuum. Orrok gives k — 350r2VV, for clean copper tubes, where V — water velocity in tubes, feet per second, r = ratio of partial pressure of steam to total pressure in the condenser. If condensers and vacuum pumps are tight, r = 0.95.
Small tubes therefore favor high transmission if the water is clean. The length of a tube should be not over 30 times its diameter, and the aggregate tube length (all passes) probably not over 50 times. A multiplicity of passes may be used, the steam striking water passages several times. If s = pitch of tubes on tube sheets (usually about 50 per cent more than the tube diameter) the number of tubes per square foot of tube sheet area is 166 -v-s2.
The surface condenser is generally used with turbines, both because it usually permits of a better vacuum and because the condensed steam, free from oil, is thereby unmixed with the (usually inferior) circulating water. In engine plants, if circulating water is salt or otherwise unsuitable for boiler feed, the condensed steam may be used for boiler feed if a surface condenser is used, but care must be taken that cylinder oil does not reach the boilers.
Condenser Auxiliaries. — Unless water is available under pressure, a circulating pump is required. The head against which it pumps is that due to difference of levels plus friction head. The quantity of water may be computed from the equation referred to: usually 20 to 40 times the weight of steam condensed for engines: 50 to 100 times, for turbines. Circulating pumps may be of either the plunger or centrifugal type, the latter being particularly adapted for this application (see under boiler feed pumps as to sizes and efficiencies).
The vacuum pump has heretofore been generally of the plunger type. Its displacement should be about 40 times that of condensed steam to be handled, to allow for air and vapor. In the horizontal independent jet condenser, the vacuum and circulating functions are combined in one cylinder. In barometric and ejector condensers, the water velocity (or the head of a column of tail water) will maintain a sufficiently good vacuum for most engines without the use of a vacuum pump. For turbine plants and high vacuum work, three pumps are generally used: one for circulating water, one for pumping out the condensed